1. Field of the Invention
The present invention relates to an Exhaust Gas Recirculation (herein after EGR) control unit and EGR control method which are for calculating a rate (actual EGR rate) of an EGR gas, refluxed into a combustion chamber of an internal combustion engine, to fresh air and further for controlling an EGR adjusting device on the basis of the calculated actual ERG rate.
2. Description of the Related Art
So far, there has widely known an EGR apparatus in which a portion of an exhaust gas from an internal combustion engine (engine) is refluxed or returned into an intake gas (fresh air) to achieve the reduction of NOx.
In such type of EGR apparatus, intake and exhaust passages of the engine are connected through an EGR passage, and at an intermediate portion of this EGR passage, an EGR valve is placed which controls the reflux quantity of an exhaust gas (EGR gas). Moreover, a lift quantity of the EGR valve is controlled to create a target EGR rate according to driving conditions, thereby controlling the reflux quantity of the exhaust gas.
In this EGR apparatus, concretely, a target opening degree (target lift quantity) of the EGR valve is read out from a map on the basis of an engine speed and a fuel injection quantity, and the actual EGR valve lift quantity is detected through the use of a sensor so that the opening degree of the EGR valve is feedback-controlled on the basis of these actual lift quantity and target lift quantity.
Meanwhile, in the recent years, there have been known various types of techniques for detecting a failure of such EGR apparatus. For example, so far, there has widely been known a technique (first conventional technique) in which a judgment is made that a failure of the EGR valve has occurred when a deviation between the actual lift quantity and a target lift quantity exceeds a given threshold at the above-mentioned feedback control (for example, see Japanese Patent Publication No. 2727534).
In addition, there has also known a technique (second conventional technique) which is designed to detect a failure of an EGR apparatus by measuring the quantity of fresh air through the use of an airflow sensor. This technique is based upon the fact that the fresh air quantity decreases by an amount corresponding to a reflux quantity of an EGR gas during the operation of the EGR apparatus, and is designed to make a judgment as to the occurrence of a failure of the EGR apparatus on the basis of a fresh air quantity at the operation of the EGR apparatus.
Still additionally, Japanese Patent Laid-Open No. HEI 9-317568 discloses a technique (third conventional technique) in which a judgment is made on the occurrence of a failure of an EGR apparatus by detecting an excess intake air ratio through the use of a xcex sensor. In this technique, for example, an O2 sensor acting as the xcex sensor is located in an intake passage existing on the downstream side of a junction between the intake passage and an EGR passage so that the actual EGR rate is calculated on the basis of the detection result thereof and a judgment as to the occurrence of a failure of an EGR valve is made by comparing this result with a target EGR rate (estimated EGR rate).
However, such conventional techniques create the following problems. For example, although the above-mentioned first conventional technique can detect a failure stemming from adherence of an EGR valve, it cannot detect failures (for example, failure of an EGR cooler or clogging of an EGR passage) other than the failure of the EGR valve.
Furthermore, in the case of the employment of the aforesaid second conventional technique, a general diesel engine, for example, is not equipped with an airflow sensor, and therefore, for applying this technique to the diesel engine, there is a need to place an airflow sensor additionally for the purpose of the detection of a failure of the EGR apparatus. Incidentally, the airflow sensor is relatively high in price, which leads to a considerable increase in cost.
Still furthermore, for the employment of the aforesaid third conventional technique, the required xcex sensor (linear O2 sensor) is expensive, which also causes a rise in cost, and there exists a problem of low reliability.
Therefore, there is a requirement for a technique which is capable of calculating an EGR rate at a low cost.
The present invention has been developed in order to meeting this requirement, and it is therefore an object of the invention not only to calculate an EGR rate at a low cost, but also to control an operation of an EGR apparatus with high accuracy, and even to securely make a judgment on a failure of an EGR apparatus on the basis of the calculated EGR rate.
For this purpose, in accordance with the present invention, there is provided an EGR control unit comprising an EGR passage for making a communication between an exhaust passage and an intake passage in an internal combustion engine, an EGR adjusting device including at least an EGR valve for opening and closing the EGR passage and drive means for driving the EGR valve, temperature detecting means detect in-passage temperature at least two passage of the intake passage, an atmospheric air introduction passage located on an upstream side of the intake passage and communicated with the atmosphere and the EGR passage, actual EGR rate calculating means for calculating a rate of an exhaust gas to an intake gas to the internal combustion engine on the basis of detection information from the temperature detecting means, and EGR control means for controlling the EGR adjusting device in accordance with the actual EGR rate calculated by the actual EGR rate calculating means.
Preferably, the temperature detecting means includes a first temperature detecting element placed in the EGR passage, a second temperature detecting element placed in the atmospheric air introduction passage, and a third temperature detecting element placed in the intake passage.
More preferably, the actual EGR rate calculating means is made to calculate the actual EGR rate R on the basis of an exhaust gas temperature Te, an atmospheric temperature Tb1 and an gas temperature Tb2 in the intake passage, respectively detected by the first, second and third temperature detecting elements, according to the following equation.
R=(Tb2xe2x88x92Tb1)/(Texe2x88x92Tb1) 
Furthermore, the temperature detecting means includes a second temperature detecting element placed in the atmospheric air introduction passage and a third temperature detecting element placed in the intake passage, and the actual EGR rate calculating means includes exhaust gas temperature estimating means for estimating an expected exhaust gas temperature on the basis of a driving condition of the internal combustion engine so that the actual EGR rate calculating means obtains the actual EGR rate as a function of an atmospheric temperature Tb1 detected by the second temperature detecting element, an intake passage gas temperature Tb2 detected by the third temperature detecting element and an expected exhaust gas temperature Texe2x80x2 obtained by the exhaust gas temperature estimating means.
In this case, preferably, the actual EGR rate calculating means calculates the actual EGR rate R according to the following equation.
R=(Tb2xe2x88x92Tb1)/(Texe2x80x2xe2x88x92Tb1) 
In addition, it is also appropriate that the exhaust gas temperature estimating means corrects the expected exhaust gas temperature Texe2x80x2 on the basis of a parameter which affects the exhaust gas temperature.
In this case, preferably, the exhaust gas temperature estimating means corrects the expected exhaust gas temperature Texe2x80x2 on the basis of the atmospheric temperature Tb1 detected by the second temperature detecting element.
Moreover, preferably, the exhaust gas temperature estimating means corrects the expected exhaust gas temperature Texe2x80x2 on the basis of a deviation between a reference outside air temperature at the time of the conversion of the expected exhaust gas temperature Texe2x80x2 into data and the atmospheric temperature Tb1.
Still moreover, it is also appropriate that the exhaust gas temperature estimating means corrects the expected exhaust gas temperature Texe2x80x2 on the basis of the actual EGR rate calculated by the actual EGR rate calculating means.
In this case, preferably, the exhaust gas temperature estimating means corrects the expected exhaust gas temperature Texe2x80x2 on the basis of a deviation between an assumed EGR rate corresponding to a driving condition of the internal combustion engine at the time of the conversion of the expected exhaust gas temperature Texe2x80x2 into data and the actual EGR rate.
Furthermore, it is also appropriate that the aforesaid temperature detecting means includes a first temperature detecting element placed in the EGR passage, a third temperature detecting element placed in the intake passage and a fourth temperature detecting element for detecting an outside air temperature at an upstream side of the atmospheric air introduction passage, and the aforesaid actual EGR rate calculating means includes atmospheric temperature estimating means for estimating an atmospheric temperature in the atmospheric air introduction passage on the basis of the outside air temperature detected by the fourth temperature detecting element and the EGR rate calculating means obtains an actual EGR rate on the basis of an expected atmospheric temperature Tb1xe2x80x2 obtained by the atmospheric temperature estimating means, a gas temperature Tb2 in the intake passage detected by the third temperature detecting element and an exhaust gas temperature Te detected by the first temperature detecting element.
In this case, preferably, the actual EGR rate calculating means calculates an actual EGR rate R according to the following equation.
R=(Tb2xe2x88x92Tb1xe2x80x2)/(Texe2x88x92Tb1xe2x80x2) 
Moreover, preferably, the atmospheric temperature estimating means corrects the expected atmospheric temperature Tb1xe2x80x2 on the basis of a parameter which affects an exhaust gas temperature.
Still furthermore, it is also appropriate that the aforesaid temperature detecting means includes a fourth temperature detecting element for detecting an outside air temperature at an upstream side of the atmospheric air introduction passage and a third temperature detecting element placed in the intake passage, and the aforesaid actual EGR rate calculating means includes an exhaust gas temperature estimating means for estimating an expected exhaust gas temperature on the basis of a driving condition of the internal combustion engine and an atmospheric temperature estimating means for estimating an atmospheric temperature in the atmospheric air introduction passage on the basis of the outside air temperature detected by the fourth temperature detecting element and the actual EGR rate calculating means obtains an actual EGR rate on the basis of an expected atmospheric temperature Tb1xe2x80x2 obtained by the atmospheric temperature estimating means, a gas temperature Tb2 in the intake passage detected by the third temperature detecting element and an expected exhaust gas temperature Texe2x80x2 obtained by the exhaust gas temperature estimating means.
In this case, preferably, the actual EGR rate calculating means calculates an actual EGR rate according to the following equation.
R=(Tb2xe2x88x92Tb1xe2x80x2)/(Texe2x80x2xe2x88x92Tb1xe2x80x2) 
In addition, it is also appropriate that the EGR control means controls the EGR adjusting device so that the actual EGR rate becomes a target EGR rate set in accordance with a driving condition of the internal combustion engine.
Still additionally, it is also appropriate that the EGR adjusting device includes EGR valve opening degree detecting means for detecting an actual opening degree of the EGR valve, and the EGR control means controls the EGR adjusting device so that the actual opening degree of the EGR valve detected by the EGR valve opening degree detecting means becomes a target opening degree set in accordance with a driving condition of the internal combustion engine and controls the EGR adjusting device on the basis of a deviation between a target EGR rate set in accordance with the driving condition of the internal combustion engine and the actual EGR rate for correcting the actual opening degree of the EGR valve.
Yet additionally, it is also appropriate that the EGR control means makes a judgment as to the presence or absence of a failure of the EGR adjusting device on the basis of a result of comparison between a target EGR rate set in accordance with a driving condition of the internal combustion engine and an actual EGR rate calculated by the actual EGR rate calculating means.
In this case, preferably, the EGR adjusting device includes EGR valve opening degree detecting means for detecting the actual opening degree of the EGR valve, and when a judgment shows the presence of a failure of the EGR adjusting device, the EGR control means makes a judgment to locate a place of the failure in the EGR adjusting device on the basis of a target opening degree of the EGR valve set in accordance with a driving condition of the internal combustion engine and an actual opening degree detected by the EGR valve opening degree detecting means.
Furthermore, in accordance with the present invention, there is provided an EGR control method of controlling an EGR adjusting device including at least an EGR passage for making a communication between an exhaust passage and an intake passage in an internal combustion engine, an EGR valve for opening and closing the EGR passage and drive means for driving the EGR valve, an actual EGR rate to an intake gas to the internal combustion engine being calculated on the basis of information on in-passage temperatures of at least two of the intake passage, an atmospheric air introduction passage located on the upstream side of the intake passage and communicated with atmosphere and the EGR passage so that the EGR adjusting device is controlled on the basis of the calculated actual EGR rate.
Preferably, the actual EGR rate is calculated on the basis of the information on in-passage temperatures of the intake passage, the atmospheric air introduction passage and the EGR passage.
More preferably, the actual EGR rate R is calculated according to the following equation on the basis of an exhaust gas temperature Te in the EGR passage, an atmospheric temperature Tb1 in the atmospheric air introduction passage and an gas temperature Tb2 in the intake passage.
R=(Tb2xe2x88x92Tb1)/(Texe2x88x92Tb1) 
In this connection, the actual EGR rate R can also be calculated according to the following equation on the basis of an expected exhaust gas temperature Texe2x80x2 based on an engine driving condition, an atmospheric temperature Tb1 in the atmospheric air introduction passage and a gas temperature Tb2 in the intake passage.
R=(Tb2xe2x88x92Tb1)/(Texe2x80x2xe2x88x92Tb1) 
Moreover, the actual EGR rate R can also be calculated according to the following equation on the basis of an exhaust gas temperature Te in the EGR passage, an expected atmospheric temperature Tb1xe2x80x2 based on an outside air temperature at an upstream side of the atmospheric air introduction passage and a gas temperature Tb2 in the intake passage.
R=(Tb2xe2x88x92Tb1xe2x80x2)/(Texe2x88x92Tb1xe2x80x2) 
In addition, it is also appropriate that the EGR adjusting device is controlled so that the actual EGR rate becomes a target EGR rate set in accordance with a driving condition of the internal combustion engine.
Still additionally, it is also appropriate that the EGR adjusting device includes EGR valve opening degree detecting means for detecting an actual opening degree of the EGR valve, while the EGR adjusting device is controlled so that the actual opening degree of the EGR valve detected by the EGR valve opening degree detecting means becomes a target opening degree set in accordance with a driving condition of the internal combustion engine and the EGR adjusting device is controlled on the basis of a deviation between a target EGR rate set in accordance with a driving condition of the internal combustion engine and an actual EGR rate to correct the actual opening degree of the EGR valve.
Yet additionally, it is also appropriate that a judgment on the presence or absence of a failure of the EGR adjusting device is made on the basis of a result of comparison between the target EGR rate set in accordance with the driving condition of the internal combustion engine and the actual EGR rate calculated by the actual EGR rate calculating means.
In this case, preferably, the EGR adjusting device includes EGR valve opening degree detecting means for detecting an actual opening degree of the EGR valve, and when the judgment shows the presence of a failure in the EGR adjusting device, a judgment for locating a place of a failure in the EGR adjusting device is made on the basis of a target opening degree of the EGR valve set in accordance with a driving condition of the internal combustion engine and the actual opening degree detected by the EGR valve opening degree detecting means.